Pin grid array (PGA) packages are well known in the art of packaging semiconductor die, particularly integrated circuits. Very large scale integrated (VLSI) circuits tend to employ PGAs for their package because the PGA provides many more leads than are available in the conventional dual in-line package (DIP) or plastic leaded chip carrier (PLCC). The DIP has leads only along two sides of the package, and the PLCC has leads around four sides of the package, in contrast to the PGA which has a large number of leads perpendicularly extending from a flat package body for a "bed of nails" appearance. Thus, VLSI integrated circuit chips which carry a relatively large number of bonding pads are often placed in PGAs which have a large number of pins relative to the package size, or the space it occupies or "footprint" on the printed circuit board (PCB) that it will ultimately be mounted upon.
Typically, a ceramic substrate is employed for the PGA package. The substrate will have a central region therein for bonding the semiconductor die thereto. A conductive pattern of metal traces runs from the central die bonding region to the pins that perpendicularly pierce the substrate and that are brazed to the substrate. The pins are brazed to the ceramic substrate before the semiconductor die is mounted thereon. The bonding pads of the die are wire bonded to the conductive pattern. A cap is then hermetically sealed to the substrate by glass or frit sealing techniques covering the entire surface of the substrate including the die, the conductive pattern, the wire bonds and the tops of the pins.
One of the advantages of the PGA package described above is that the semiconductor die or integrated circuit chip is hermetically sealed into the package. However, one of the major disadvantages of the PGA package is that they are very expensive and can cost as much as or more than the semiconductor die itself. This is particularly true if a multilayer ceramic substrate is used that employs layers of interconnecting patterns between ceramic layers. Another expensive part of the PGA package is that the conductive patterns therein are gold and the pins that are brazed in before the die bonding step are gold as well. This extensive use of gold drives up the cost of the package. Another problem with the conventional PGA is that the testing of the part is performed after the die is assembled in this expensive package. If the die tests bad, then not only is the die scrapped, but the expensive package must be scrapped as well.
Attempts have been made to reduce the cost of ceramic PGAs while still providing hermeticity for the die. One approach has been to use copper pins instead of gold plated copper pins. However, a problem with this approach is that after the copper pins are silver brazed to the substrate, intermetallic shorting connections form between the pins. These dendritic growths occur upon the application of bias under humid conditions.